Adding already animated armature to a model
In principle you have to bring your mesh into the position that it matches the given armature. One way would be to create your own armature, position your mesh so it alligns with the downloaded armature and press "Apply" in the Modifier tab.

But probably the Sculpt Brush would be a better solution for you: https://developer.blender.org/D5647

Remember to make a backup of your file before you start modifying the mesh.